This research concerns the coupling role played by calcium in the secretion of renin from the kidney. Rat renal cortical slices are used. Transmembrane Na gradient, via operation of a putative Na-Ca exchange mechanism, affects intracellular Ca (Cai). Na gradient is reduced by increases in Nai (achieved by inhibition of Na, K-ATPase using low Ke, ouaban or vanadate) or decreases in effects on renin secretion. Na gradient is increased by decreasing Nai (by stimulating Na, K-ATPase using phenytoin, isoproterenol). Phenytoin and isoproterenol stimulate secretion and this can be blocked by agents which increase Cai Depolarization of the membrane (with high K) inhibits secretion in a Ca-dependent manner; the effect is blocked by D-600, a specific Ca-antagonist. Other Ca-antagonists are being studied, and their interaction with Cae. Peptides such as angiotensin II and antidiuretic hormone inhibit secretion in a Ca-dependent manner. The possibility that they act by depolarization, is being assessed by trying to block the effects with Ca-antagonists. The findings fit the hypothesis that Ca plays an inhibitory coupling role in renin secretion.